1. Field of the Invention
The present invention is useful in the field of manufacturing printed circuit boards and more particularly in the electrical discharge manufacture of a pattern, such as a mold face for forming plastic circuit board substrates.
2. Brief Description of the Prior Art
A number of different manufacturing methods have been employed in the manufacturing of printed circuit boards. Generally, the modern circuit board has evolved to a dielectric substrate or insulating panel having a surface plated with a conductive material. The desired circuit pattern has been provided by a number of methods such as chemical etching with nitric or other mineral acids, cutting, debossing or molding with a mold face. Various molding methods have been employed including injection molding, transfer molding and mechanical routing. A particularly desirous approach is to form the desired design by etching or milling a steel plate and then utilizing the resultant mold face for forming the insulating panels or substrate of the circuit boards. A method of making a pressure die by sandblasting for a mechanical joining of a foil to a substrate is shown in U.S. Pat. No. 2,925,645.
Broadly, in the field of cutting mold faces, electrical discharge machines have been utilized wherein a highly conductive workpiece and a conductive electrode are attached to opposite terminals. The workpiece and the electrode are brought together for an electro-erosion of the conductive workpiece generally in a dielectric medium. The following U.S. Pat. No. 3,469,058 and U.S. Pat. No. 3,167,632, are cited of general interest in this field.
In producing relative intricate mold faces it is frequently necessary to utilize a rough electrode tool for an initial electro-erosion at a relatively high current flow. A subsequent electrode is utilized for a final finishing step of electro-erosion at generally lower current flow.
One approach has been to utilize a graphite substrate with a desired pattern cut into the face of the graphite substrate as a roughing tool or electrode. For the finishing step, a copper electrode is generally required to produce an adequate steel mold face. The high labor requirements and cost in producing a steel mold face have limited production of molds by an electrical discharge procedure to those items that can justify the cost. The relatively high cost of producing the steel die requires a significant number of molded items to be produced to amortize the cost and make the end product competitive with other methods of producing the same product.
As mentioned above, steel molds for printed circuit boards are to the best knowledge of the inventors, commercially produced today by etching or various forms of milling or cutting, such as a pantograph copy milling machine disclosed in U.S. Pat. No. 3,886,847. It is not believed that an electrode discharge process has previously been utilized to form mold faces for a printed circuit board.